1. Field of the Invention
The present invention relates to a focus detection method and a focus detection device, wherein the focus adjustment condition of the photography lens is detected in a plurality of focus detection areas.
2. Description of Related Art
A focus detection device has been known in which an arbitrary focus detection area can be selected from among a plurality of focus detection areas provided on the photography screen. For instance, in auto focus cameras, several focus detection areas are provided in the center of and along the periphery of the photography screen, the photographer can select an arbitrary focus detection area using an operation part provided on the camera body, and the focus adjustment condition of the photography lens is detected in the selected focus detection area.
A charge accumulation type image sensor such as a CCD or the like is generally used as the light receptor in the aforementioned focus detection device.
When focus detection is performed using a charge accumulation type image sensor, it is necessary for the subject image signal output from the image sensor to be at an appropriate level with respect to the processing circuit that processes it. For instance, when the subject image signal is processed digitally by a microcomputer, if the subject image signal is not kept at a certain level without exceeding the A/D conversion input range of the A/D converter built into the microcomputer, the quantization errors can occur in A/D conversion, causing deterioration of the focus detection accuracy.
In order to keep the subject image signal at a suitable level over the range of brightness from dim to bright light, the accumulation time of the charge accumulation type image sensor is controlled. For instance, if the current accumulation time Tn is determined, as shown in equation 1, using the target peak value Px and the peak value Pn-1 and accumulation time Tn1 of the previous subject image signal, the subject image signal can be maintained at a suitable level for focus detection computation without exceeding the A/D conversion range. EQU Tn=Tn-1*Px/Pn-1 (1)
However, the focus detection device described above has the following problems.
When the focus detection area is changed, in order to initialize the charge accumulation type image sensor corresponding to the newly selected focus detection area and keep the subject image signal output from this image sensor at a level suitable for focus detection computation, it is necessary to repeat the accumulation time control cycle of equation (1) numerous times.
Because no previous subject image signal data exists after initialization, image sensor charge accumulation is performed by setting an accumulation time corresponding to a certain preset brightness. However, because the brightness of the subject can change by a factor of nearly 2 to the 20th power, when there is a large difference between the actual subject brightness and the set brightness, the peak value of the subject image signal obtained is too high, the numerical value becomes 0 through quantization, and computation of the next accumulation time using equation (1) becomes impossible. In this case, the control process of setting the next accumulation time as a fraction of the present accumulation time when the peak value of the subject image signal is too high and setting the next accumulation time as a multiple of the present accumulation time when the peak value of the subject image is below a preset value must be repeated, and only when the peak value of the subject image signal falls within a preset range can equation (1) finally be applied.
Accordingly, there is a delay in the output of focus detection results when the focus detection area is changed, and because of this, focus detection responsiveness is poor.
In order to improve the focus detection responsiveness when the focus detection area is changed, focus detection can be performed at all times even in image sensors that correspond to focus detection areas that have not been selected. However, A/D conversion and focus detection computations are performed with a single microcomputer in order to keep costs down, and if it is necessary to perform focus detection computation and A/D conversion on subject image signals output from image sensors for focus detection areas not selected in addition to focus detection computation and A/D conversion of subject image signals output from the selected focus detection area, the time required for the focus detection cycle would increase, and consequently, the responsiveness of the normal focus detection action would decline.